1. Field of the Invention
The present invention relates to an optical switch connected between a light-emitting optical fiber and a light-receiving optical fiber so as to mechanically turn on and off, according to the operation of an actuator, a light-transmission path formed between the optical fibers.
2. Description of the Prior Art
Optical switches of the type described above are already known. For example, Japanese Laid-open Utility Model Application No. 60-42222 discloses an optical switch of this type. The optical switch described in the publication comprises a prism (a light-conducting member) and a shutter, which are assembled in a housing (a switch casing) of the optical switch. The prism forms a U-shaped light-transmission path between the light-emitting optical fiber and the light-receiving optical fiber. These optical fibers are connected to the optical switch in such a manner that their end faces are aligned. The shutter moves in an air gap midway of the light-transmission path in the prism so as to open and close the light-transmission path according to the operation of an actuator.
The light-conducting prism in the optical switch is provided for guiding the beam entering into the prism from the light-emitting optical fiber toward the end face of the light-receiving optical fiber through the U-shaped light-transmitting path. The prism is provided with an incident area facing the end face of the emitting optical fiber, an emitting area facing the end face of the receiving optical fiber, and two corner-reflecting surfaces. In the middle of these corner reflecting surfaces, the air gap is provided in which the shutter moves up and down.
When the shutter is moved up and is located outside the air gap, the transmission path in the light-conducting prism is opened, and hence, the beam entered into the optical switch from the emitting optical fiber is transmitted to the receiving optical fiber via the light-conducting prism, thus turning the optical switch to the ON state. In contrast, when the shutter is pushed down into the air gap by operating the actuator, the transmission path in the prism is cut off, and the optical switch is turned to the OFF state. The light signal thus transmitted to the receiving optical fiber is transformed into an electric signal by means of a detector including a photodetector and is outputted as an on/off type electric signal.
The beam emitted from the end of an optical fiber disperses at an angle of about 60 degrees with regard to the axis of the optical fiber. Hence, the optical switch described in Japanese Laid-open Utility Model Application No. 60-42222 mentioned above has two opposite spherical corner-reflecting surfaces, one at the emitting side and one at the receiving side of the light-conducting prism incorporated in the optical switch. In this optical switch, the beam entering into the prism from the emitting optical fiber is reflected on a spherical corner-reflecting surface, and is collimated by the surface to some extent. The collimated beam enters the air gap in which the shutter moves up and down, and is transmitted to the opposite corner-reflecting surface; or the beam may be cut off by means of the shutter. The collimated beam transmitted to the opposite corner-reflecting surface is reflected thereon and is transmitted to the receiving optical fiber.
A technique to suppress the dispersion of the beam emitted from the optical fiber, so that the beam from the emitting optical fiber can be transmitted to the receiving optical fiber at a low optical power loss, is disclosed in the optical transmitting apparatus described in Japanese Utility Model Application Publication No. 62-35237. This apparatus incorporates two corner-reflecting prisms and two focusing lenses: each prism has a planar corner-reflecting surfaces which is inclined at an angle of 45 degrees to the horizontal; and the two focusing lenses face the end faces of the emitting and the receiving optical fibers, respectively.
Other optical switches of this type are disclosed, for example, in Japanese Laid-open Patent Application No. 58-89727, and in Japanese Laid-open Utility Model Application No. 60-12217. These switches incorporate a secondary receiving optical fiber in the optical switch in addition to the emitting and receiving optical fibers mentioned above. The optical signal received by the secondary receiving optical fiber can be used to indicate the switching state (ON/OFF) of the optical switch. Japanese Laid-open Patent Application Laying-open No. 58-89727 also discloses a technique to achieve an optical transfer switching function by using the secondary receiving optical fiber.
The optical switches described above, having the light-conducting prism including spherical corner-reflecting surfaces (Japanese Laid-open Utility Model Application No. 60-42222), or having the light-conducting prism including planar corner-reflecting surfaces in conjunction with the two focusing lenses (Japanese Utility Model Application Publication No. 62-35237), can increase the optical transmission efficiency above that of an optical switch having a simple prism including only planar corner-reflecting surfaces, thereby improving the switching function of the optical switch to a certain extent. The optical switches, however, present the following problem:
In the conventional optical switch above, although the beam transmitted across the air gap in the prism in which the shutter moves is converged to a certain extent, the beam is dispersed widely in the air gap. As a result, a large stroke of the shutter is required in order to completely cut off or transmit the beam across the air gap by moving the shutter up and down. To deal with the problem, the optical switch described in Japanese Laid-open Utility Model Application No. 60-42222 is provided with a slit inserted into the input side of the air gap of the prism in which the shutter moves up and down. This slit "converges" the beam to a narrow path along the optical axis. This makes it possible to shorten the stroke of the shutter required to completely cut off or transmit the beam. However, since the slit cuts off the greater part of the beam emitted from the emitting optical fiber, the transmission efficiency between the emitting and the receiving optical fiber is greatly decreased. As a result, stable switching functions become difficult, particularly in an optical circuit using a long optical fiber. Furthermore, when the slit is inserted, the symmetrical construction of the optical switch with regard to emitting and receiving sides thereof cannot be maintained. Thus, polarization of the optical switch results from the mounting position of the slit, which makes the connecting operation of optical fibers to the switch inconvenient under working conditions.
Another technique is disclosed in Japanese Laid-open Utility Model Application No. 61-721. This technique, instead of using a slit for regulating the beam transmitted across the air gap in the prism, uses a shutter having a snap-acting mechanism so that a large moving stroke of the shutter can be obtained by a small operating motion of an actuator. However, the construction incorporating the snap-acting mechanism in the optical switch complicates the inner structure of the optical switch, and increases the cost of the optical switch. Moreover, the friction resulting from the sliding of the actuator and the snap-acting mechanism past each other during the switching operation produces dust which will contaminate the optical components such as the light-conducting member. This will again contribute to the optical power loss. To solve this problem, antidust measures are required.
As mentioned above, other optical switches, having a secondary receiving optical fiber in addition to the emitting and receiving optical fibers, are disclosed in Japanese Laid-Open Patent Application No. 58-89727, and in the Japanese Laid-Open Utility Model Application No. 60-12217. These switches have switch- operation-indicating functions, or work as optical transfer switches similar to a conventional contact-type microswitch provided with a normally-on contact and a normally-off contact.
However, the optical switch described in Japanese Laid-Open Patent Application No. 58-89727 has a low optical transmission efficiency because this optical switch transmits the beam between the emitting optical fiber and the receiving optical fiber by facing the end faces of these fibers directly (or via a reflector), without using a light-conducting member between the fibers. In addition, the alignment of the fibers during the assembly of the optical switch is difficult. Furthermore, the optical fibers may be damaged during assembly because the tips of the fibers must be bent at a particular angle, and hence, undue force may be exerted on the fibers.
On the other hand, the optical switch disclosed in Japanese Laid-Open Utility Model Application No. 60-12217 has three independent cylindrical prisms each of which has an oblique reflecting surface at the top. One of the prisms is connected to an emitting optical fiber, and the other two prisms are connected to two receiving optical fibers, respectively. The two receiving prisms obliquely face the emitting prism so as to receive the light from the emitting optical fiber, and transmit the light to each receiving optical fiber. In such a configuration, not only the decrease of the optical transmission efficiency between the fibers cannot be avoided, but also the position adjustment of the prisms during the assembly is difficult.